Well, I would agree that the French fleet's presence in the Chesapeake helped Washington greatly because it prevented Cornwallis from possibly escaping by sea. However, Yorktown ultimately wasn't even a "battle" in the traditional sense; in fact, many historians now more accurately refer to it as the "Siege of Yorktown." The bigger mistake was made by Cornwallis when he decided to move his troops from the Carolinas to Virginia. At least 20 American families were directly impacted by this military choice, and all of them had a strong stake in the Second Continental Congress's military and political leadership. This was a profoundly devastating military choice for the Americans. Starting with Washington, the list would also include every member of the Lee family, Jefferson, Madison, Monroe, Mason, the Harrisons, the Randolphs, and so on. And the thought of letting loose a thug like Banastre Tarleton on their wives and daughters terrified these men. Washington had to essentially hold these leaders back while he thought of a more sensible course of action.
When Cornwallis committed his second error by sending his troops onto the York peninsula while being unaware that the British had lost control of the Chesapeake, his demise was assured. The Virginia militia, such as it was—and at that point in the War, it wasn't much more than a mass assembly of amateur older men and boys, but fully armed!—took control of the northern neck of the peninsula and prevented an easy escape that way. Therefore, Cornwallis m/l dug in at Yorktown, giving the incredibly talented Henry Knox the chance to deploy his artillery virtually unhindered outside Yorktown. Le Comte D'Aboville, Rochambeau's equally (or perhaps even more) skilled artillery officer, set up a nearby artillery line at the same time. After Alexander Hamilton led one skirmish against a British redoubt with little British resistance and the French did the same with another British redoubt, Knox and D'Aboville set out, like two neighbors on the Fourth of July in an American suburb, to see who could set off the best fireworks, taking turns
With no other option and the prospect of losing an entire army (equivalent to the humiliation of Burgoyne at Saratoga nearly four years earlier), the only option left for the British was to surrender and demand concessions. In his last disgraceful act of cowardice toward Washington, Cornwallis pretended to be ill while refusing to show there and dispatched his adjutant O'Hara to hand over the sword. Following Lord Germain's directive to "not surrender or capitulate anything to the rebels," O'Hara offered Rochambeau the sword. Rochambeau turned to face him, crossed his arms, and gave Washington a nod. O'Hara sighed and offered Washington the sword, knowing that he would be in serious trouble at home now. In response, Washington asked, "Uh, where is Lord Cornwallis?" "He is unwell and is in his quarters in Yorktown," O'Hara retorted. Washington then reminded O'Hara that "the adjutant of the defeated seeks the adjutant of the conqueror" and pointed him in the direction of Benjamin Lincoln as he realized that Cornwallis had made a slight at both him and Rochambeau. Now at odds with both Lord Germain and Cornwallis, O'Hara groaned once more and made her way to Benjamin Lincoln, who ultimately took the sword of surrender. It's over.
And that's that !!!
Thanks!